Migrants by the side of a road in Calais as French ferry workers strike.Migrants by the side of a road in Calais as French ferry workers strike.Migrants queue for food handouts at a migrant camp in Calais, France, as the migrant crisis across Europe continues to escalate.Migrants camp in in Calais, France, as the migrant crisis across Europe continues to escalate.Striking ferry workers burn tyres as they block a ramp leading into the Eurotunnel before being dispersed by riot police in Calais

The Channel Tunnel has been shut and services through it suspended after migrants caused chaos in Calais by attempting to board UK-bound trucks.

Eurotunnel's through-tunnel shuttle services were suspended, while high-speed train company Eurostar announced mid-afternoon that services were being cancelled for the rest of the day.

Three trains stuck outside the tunnel on the English side had to return to St Pancras in London while other trains on the French side were taken back to Paris and Brussels.

Eurostar advised passengers not to go to stations.

The trouble began as migrants tried to to exploit wildcat strike action by French ferry workers.

Lorry drivers forced to slow or stop due to queuing traffic on approaches to the northern French port were faced with migrants attempting to clamber on board.

Migrants were seen walking by the sides of motorways looking for opportunities to jump on to lorries heading to the Port for Dover.

More problems ensued as French ferry workers striking over fears they will lose their jobs next month then trespassed on to Eurotunnel tracks, forcing services to be aborted.

High-speed train company Eurostar said: "They have broken into the tunnel and the tunnel is shut."

Kent Police said the industrial action is set to last until this evening.

Today's problems come amid a worsening migrant situation near the Port of Calais where numbers camped there have swelled to more than 3,000 since April.

Aid workers have reported a "catastrophic" situation, with predictions that some 2,000 more migrants displaced from war-torn countries including Eritrea, Syria and Afghanistan could arrive over the summer.

British haulier Chris Cary spoke of how he has been stuck in Calais for seven hours, and told of the brazen attempts by migrants to try to get on to lorries bound for the UK.

He told Sky News: "It is a bit of a mess. I have been here since 3am. The industrial action started about 4.45am.

"I shot off down to the train thinking I would get the train, and I couldn't get on the train, so I've come back to the boat and I've been here seven hours so far.

"When I was down at the train, (migrants) were trying your doors to get in the cab, trying to sneak under the axles on the trailers, trying to cut the locks and seals on the back of the trailers, even trying to climb on the roof of the trailers.

"Any way they were trying to get in to get to England. This morning was the worst I had ever seen it because there was no police presence."

A Port of Dover spokesman said: "Due to a period of industrial action in France commencing at 0350 BST on June 23, the Port of Calais continues to remain closed.

"All ferry services to and from the Port of Dover to Calais remain affected, with DFDS Seaways running a shuttle service to Dunkirk."

The Foreign and Commonwealth Office (FCO) updated its travel advice on France, urging people to keep vehicle doors locked in slow-moving traffic and when unattended in Calais.

It said: "There are large numbers of illegal migrants in and around Calais, who may seek to enter the UK illegally.

"Although local police patrols have been reinforced, you should keep vehicle doors locked in slow-moving traffic and secure your vehicle when it is left unattended."

There were calls for the French authorities to intervene to ensure the free movement of goods and people across the EU.

Helen Deeble, chief executive of P&O Ferries, which operates between Dover and Calais, said: "Our services have been stopped in Calais as a result of the decision by Eurotunnel to charter and then sell two of their ships to DFDS, at short notice and with neither party appearing to engage constructively with the French sea-going and shore staff at MyFerryLink."

She went on: "While we have tremendous sympathy for the difficult situation of these employees who fear that their jobs will end on July 2, in disrupting the operations of the whole of the port of Calais, Eurotunnel continues to operate and DFDS sails into Dunkirk.

"As a large and long-standing employer in Calais, we are bearing the brunt of the disruption. We call upon the French authorities to ensure the free movement of goods and people across EU borders and to ensure the safety of staff and customers in and around the port of Calais."

As the migrant crisis worsens, truckers were being advised not to stop within about 60 miles (97km) of Calais, to stick with other drivers and make sure padlocks are kept on vehicles.

Donald Armour, the Freight Transport Association's international manager, said: "We have heard that the strikers have set up a blockade on the A16 with burning tyres and hay bales, bringing the main motorway to a halt.

"It costs about £50 an hour to have a lorry stuck in traffic. If you're a company with half a dozen lorries, it adds up to a lot of money being lost.

"It's their worst nightmare and not a good day to be travelling." Mr Armour said the FTA was "really, really worried" about someone getting seriously hurt.

"There is a lot of fighting between the migrants who all want to be on the best part of the road to get on to the lorries," he said.

"We haven't had a fatality but it's not good. The situation is definitely worse than it has ever been."

The strike action started in Calais at around 3.50am. Port of Dover officials said the situation was being monitored closely.

Operation Stack - where freight traffic is held on the coastbound carriageway of the M20 in Kent - was implemented as queues built up on approaches to the British port. It is set to continue into tomorrow to clear traffic jams.